The invention relates to a precision high frequency integrated circuit attenuator, which can be used in various integrated circuits, such as a window comparator circuit of the type which indicates whether an input voltage is within or outside of a predefined range or an analog-to-digital converter.
FIG. 1 shows a typical window comparator circuit including a comparator 1 having a non-inverting (+) input connected by conductor 4 to a first reference voltage V.sub.REF1, its inverting (-) input connected to conductor 3 to an input signal V.sub.IN, and its output connected by conductor 6 to produce an output signal V.sub.01. A second comparator has its non-inverting input connected to conductor 3, its inverting input connected by conductor 5 to a second reference voltage V.sub.REF2, and its output connected by conductor 7 to produce an output voltage V.sub.02. If V.sub.IN is between V.sub.REF1 and V.sub.REF2, comparator 1 will not invert, so V.sub.01 will be high. Comparator 2 will not invert, so V.sub.02 will be high. If V.sub.IN is less than V.sub.REF2, comparator 2 inverts, so V.sub.02 will be low. If V.sub.IN is greater than V.sub.REF1, comparator 1 will invert, so V.sub.01 will be low.
Several difficult problems arise if a high speed window comparator of the general type shown in FIG. 1 is to be integrated onto a single monolithic integrated circuit chip. For typical standard integrated circuit manufacturing processes, the permissible range for the input signal V.sub.IN which can be applied directly to electrodes of integrated circuit transistors without causing undesired forward biasing of PN junctions and/or undesired PN junction breakdown is quite limited, typically between ground and -3 to +3 volts for present high speed IC comparators. If resistive voltage dividers are used to attenuate the input signal and reference signal before they are applied to the inputs of the integrated circuit comparators, the frequency response of the circuit is very poor because parasitic capacitors of integrated circuit resistors such as 25 and 26 in FIG. 2 usually are proportional to the resistor values and respond much differently to rapidly rising and rapidly falling edges of V.sub.IN than do the resistors 13 and 14. The result of this is unacceptable inaccuracies in the attenuation of V.sub.IN. For example, if resistors 13 and 14 are ordinary nichrome resistors and if the resistance of resistor 13 is five times that of resistor 14, the parasitic capacitance associated with resistor 13 is likely to be much larger than the parasitic capacitance 26 associated with resistor 14. Therefore, capacitive voltage division of V.sub.IN across parasitic capacitances 25 and 26 occurs in the opposite sense to voltage division of V.sub.IN across resistors 13 and 14, causing substantial errors in the attenuation of V.sub.IN from conductor 3 to conductor 15 for high frequency components. This, of course, causes inaccuracy in the results produced by the window comparator circuit 100.
Thus, there is an unmet need for an improved integrated circuit attenuator that can be used in an integrated circuit wherein accurate attenuation of both low frequency and high frequency input signals is needed, and especially where the amplitude of the input signals exceed the voltage tolerance capabilities of the internal integrated circuit which receives the attenuated signal.